Hot Workability of 16Cr-5Ni Stainless Steel Using Constitutive Equation and Processing Map

Hot Workability of 16Cr-5Ni Stainless Steel Using Constitutive Equation and Processing Map

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ScienceDirect Materials Today: Proceedings 5 (2018) 17213–17222

www.materialstoday.com/proceedings

AMPCO-2017

Hot Workability of 16Cr-5Ni Stainless Steel Using Constitutive Equation and Processing Map Brij Kishora*, G. P. Chaudharib, S. K. Nathc a Guest Faculty Department of Applied Mechanics, Motilal Nehru National Institute of Technology Allahabad 211004 India b Associate Professor, cProfessor Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee 247667 India,

Abstract The hot working characteristics of 16Cr-5Ni stainless steel are studied using the results of hot compression tests. Hot compression tests were performed to a total true strain of 0.69 in the strain rate range of 0.001-10 s−1 and temperature range of 900-1100°C. Average value for activation energy is obtained as 403 kJmol−1. The processing maps were constructed based on dynamic materials model at true strain of 0.5. Stable and unstable regimes for hot working are identified and these are verified from the microstructural observations. Optimum hot working conditions associated with dynamic recrystallization are found to be in the strain rate range of 0.01-0.1 s−1 and temperature range of 1000-1100°C, which correspond to a peak power dissipation efficiency of 31%. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017). Keywords: Thermomechanical processing; Optical microscopy; Stainless steel; Dynamic recrystallization

1. Introduction The 16% chromium and 5% nickel (16Cr-5Ni) martensitic stainless steel (MSS) is used in under- water parts of hydro-turbines. Hydropower plants prefer this material due to its good corrosion resistance and satisfactory weldability [1]. Mechanical properties of stainless steels are usually enhanced by modification of microstructure obtained through heat treatment and/or hot working [2]. Therefore, it is relevant to ascertain optimum working

* Corresponding author. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Advances in Materials & Processing: Challenges & Opportunities (AMPCO-2017).

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parameters and the characteristics of 16Cr-5Ni MSS after the TMP in order to improve their mechanical properties and erosion resistance. During TMP, temperature, strain, and strain rates play important roles. Identifying suitable TMP parameters can restrict development of deformation defects, by identification of softening mechanism, which reduces the defect formation. In recent studies the characterization of MSS after the TMP are discussed using the constitutive equations and processing maps [3-5] and optimized the TMP conditions. The work hardening and softening mechanism such as dynamic recovery (DRV) and dynamic recrystallization (DRX) influence the plastic deformation behaviour of metals under the various temperature regimes [6]. To determine the safe processing conditions and plastic instabilities, processing maps that are fully based on dynamic materials model (DMM), which follows the continuum criteria were developed [7]. With improvement in principles of irreversible thermodynamics, Prasad and other researchers applied them to TMP of a wide range of materials [8-9]. These are needed because these are able to identify the stable or instable regimes. Thus, in present work the hot compression of 16Cr-5Ni MSS is performed to characterize its TMP behavior using constitutive equations and DMM based processing maps. Applied strain rate ranged from 0.001 to 10 s−1 and temperature ranged from 900 to 1100°C, for a total of 50% deformation i.e. true strain of 0.69. The experimental results are used to determine the different materials constants and activation energy (Q) through constitutive equations described elsewhere [10]. Flow stress and the value of Z parameters are developed for an entire TMP conditions. Processing maps based on DMM are developed at true strain of 0.5. Optimal TMP conditions were identified and corresponding microstructural evolution is studied in order to validate the stable and unstable behaviour. 2. Experimental Specimens for compression test were prepared as per ASTM E 209 [11] from Ingots of 16Cr-5Ni MSS supplied by M/s. Vaishnav Steels Pvt. Ltd., Muzaffarnagar India. Specimens had diameter of 10mm and height of 15mm. The chemical composition of 16Cr-5Ni MSS is shown in Table 1 (in wt. %) was determined by using Thermo Jarrell Ash spark emission spectroscope. TMP was performed using hot compression tests, which are carried out on Gleeble® 3800 simulator. To monitor the true temperature during the TMP, K type thermocouple was spot welded on the centre of specimen along its length. Prior to test, graphite lubricant with tantalum foil of 0.05mm thick was employed to reduce the friction and sticking between the specimen and the compression anvil. Table 1. Chemical composition (wt. %) of 16Cr-5Ni MSS. C

Si

Mn

P

S

Cr

Ni

Mo

Cu

V

Al

0.062

1.06

0.82

0.023

0.007

15.53

5.15

0.83

0.12

0.05

0.090

In all the specimens, heating rate of 5°C/s was used to attain the homogenization temperature of 1100°C and the specimen were held at this temperature 120s., after which they were cooled to specific deformation temperature at the rate of 1°C/s. Before deformation, 30s delay was used to eliminate thermal gradients. The specimens were compressed by 50% and immediately air quenched. The processed specimens were cut in the centre along the compression axis, and the cut surface was observed using a Leica DMI 5000M light optical microscope. To reveal the microstructure, specimens were electro-etched using 60% nitric acid + 40% water solution. Average prior austenite grain size was measured using the linear intercept method according to ASTM E112-88 [12]. Material constants and processing conditions for safe and unstable flow determined from the true stress-strain data for true strain of 0.5, obtained from the compression test. Flow stress data were used to determine the value of Q and Z through the constitutive equations described elsewhere [10].

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3. Result 3.1. True stress-true strain flow curves for 16Cr-5Ni MSS True stress-true strain flow behaviour of 16Cr-5Ni MSS in the temperatures range of 900 to 1100°C and at strain rate of 0.01 s−1 and 1 s−1 are shown in Figs. 1a and 1b, respectively. The flow stress shows the increasing trend with strain rate and decreasing trend with temperature. A peak stress is observed in all the processing conditions. Before attainment of peak stress there is work hardening stage, which varies in each condition corresponding to the true stain value. It is observed that large work hardening occurred at low temperatures of 900 and 950°C (Fig. 1a) and at higher strain rate conditions for all temperatures (Fig. 1b).

Fig. 1. True stress-true strain curves for 16/5 MSS deformed in the temperature range of 900-1100°C with strain rates of (a) 0.01 s−1; (b) 1 s−1, respectively. The extent of work hardening strongly depends on dislocation density and weakly on the formation of sub grain boundaries [13]. For strain rate of 0.01 s−1 (Fig. 1a), after appearance of peak flow stress, there is commencement of flow softening stage and subsequently a steady state is observed in the deformation temperatures range of 10001100°C, which indicates occurrence of DRX [14-15]. However, the steady state is absent in the specimens processed at 900 and 950°C with strain rate of 0.01 s−1 (Fig. 1a) and for all processing temperature range of 9001100°C with strain rate of 1 s−1 (Fig. 1b). This behaviour is influenced by the effect of DRV or existence of flow instability i.e. flow localization or shear bands [14] in the specimens, which indicates that work hardening effect is stronger than the dynamic softening i.e. DRV or DRX [16]. 3.2 Determination of material constant for 16Cr-5Ni MSS The average slopes of the plots of versus ln and versus σ, shown in Fig. 2a and 2b respectively, give the respective values of and β. The average slopes obtained from plots are 7 and 0.06. These correspond to the values of and β, respectively at true strain of 0.5. The stress multiplier α is measured as = 0.0082 MPa−1. Average slope obtained from plots between and ln (sinh (ασ)) given in Fig. 2c correspond to the value of n. It is calculated as 5.2. The average of slopes of plots between the ln(sinh (ασ)) and 1/T (Fig. 2d), give a value of 9. Q is calculated by multiplying this average slope value with nR and its value is 408 KJmol−1. This procedure is repeated for true strain range of 0.1 to 0.69 and corresponding values of materials constants and Q are plotted and shown in Fig. 3. All material coefficients (i.e. β, n and α) show a decreasing trend from true strain range 0.1 to 0.69 (Fig. 3a-c). From Fig. 3d, the average value of Q is obtained as 403 kJmol−1 and its value ranged from 395 to 413 kJmol−1. Initially, a

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higher value of Q is obtained i.e. 395 kJmol−1. This is because in the beginning (at lower values of true strain) of the deformation, higher energy is imposed to initiate the plastic deformation. Subsequently at true strain 0.2, it decreases to 391 kJmol−1 due to initialization of softening mechanism. After this it shows a rising trend with increase in true strain values. Generally, the value of Q for the TMP is a function of alloy composition. Its value also reflects the level of energy barrier for thermal diffusivity of atoms during the hot working. It is suggested that higher values of Q represents the harder plastic deformation of the specimens during the TMP. The value of Z is calculated from the following relation [10] Z = ε exp

Q RT

Where, f σ =

=f σ .

(1)

Aσ Bexp βσ , C sinh ασ

preferred for low σ level preferred for high σ level Wide range of T and ε

(2)

Fig. 4 shows the linear fit for a plot of values of log Z and hyperbolic sine function. The intercept obtained in Fig. 4 is lnC and is equal to 35.2. Therefore, the value of C is determined as 1.99x1015.

Fig. 2. Plots of 16Cr-5Ni MSS for calculating the value of (a)

; (b) β; (c) n; and (d) lnC, for true strain at 0.5.

The sinh function for the 16Cr-5Ni MSS is derived by substituting the values of materials constants n, C and α, into equations (1) and (2) [10].The constitutive equation thus obtained is given by: Z = ε exp

.

T

= 1.99Χ10

sinh 0.0082σ

.

(3)

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Fig. 3. Plot of values of material coefficients (a) β; (b) n; (c) α; and (d) variation of activation energy Q, for 16Cr5Ni MSS in the true strain range of 0.1-0.69

Fig. 4. Zener-Hollomon parameter, Z, for 16Cr-5Ni MSS, as a function of the flow stress for hyperbolic sine function at true strain of 0.5.

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3.3 Processing map for 16Cr-5Ni MSS Variation of ɳ corresponding to the processing temperature and strain rate represent the power dissipation map as given in Figs. 5a and 5b at true strain of 0.2 and 0.5, respectively. The numbers on the contour lines in the maps represents the values of ɳ. The contour lines obtained at true strain of 0.2 and 0.5 express the similar features, which indicate that the strain does not influence the processing conditions. Therefore, the power dissipation map obtained at true strain of 0.5 is presented. The values of ɳ increases from upper left corner to the centre of the map. The highest values of ɳ obtained at true strain of 0.2 is about 28% (Fig. 5a) and it is about 31% at true strain of 0.5 (Fig. 5b). It is reported that better workability attributed to higher value of ɳ indicates the optimum condition for TMP [17-19]. Contour plot representing the variation of m for 16Cr-5Ni MSS is shown in Fig. 6a for fixed true strain of 0.5. In this study, maximum value of m is found to be 0.18 which corresponds to conditions in the strain rate range of 0.01-0.2 s−1 and temperature range of 1000- 1100°C (Fig. 6a). The second highest value of m is 0.16, which is observed in the strain rate range of 0.01-0.3 s−1 and temperature range of 975-1100°C.

Fig. 5. Distribution of power dissipation for 16Cr-5Ni MSS at true strain of (a) 0.2; and (b) 0.5.

Fig. 6. Contour map of 16Cr-5Ni MSS showing (a) the variation of m-values; and (b) processing map obtained for a fixed true strain of 0.5.

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The contour indices in Fig. 6b represent the constant ɳ and shaded region represent the instabilities in which the value of instability parameter is found to be negative. Extent of instability increases as the m values decrease (Fig. 6). Lower m value regimes exist at low temperature and high strain rate conditions and correspondingly more shaded region represents the instability. Maximum value for ɳ obtained at true strain of 0.2 is about 28% (Fig. 5a) and at true strain of 0.5 it is about 31% which shows that better workability can obtained at true strain of 0.5. This characteristic suggests the occurrence of DRX in the materials during the deformation which can enhance the intrinsic workability. Thus, DRX can be obtained in the strain rate range of (0.01-0.1 s−1) and temperature range of 1000-1100°C. At high strain rates and low temperatures two instability regions are observed. First one lies in the strain rate range of 0.3-10 s−1 and temperature range of 900-1020°C. The second lies in the strain rate range of 0.510 s−1 at 1100°C. In the instability region the TMP is considered as unsafe, therefore these conditions are avoided during the TMP of steels. 4. Discussion on workability of 16Cr-5Ni MSS A single peak followed by slow softening and subsequently attainment of a steady state is seen in the Fig. 1, which suggests the DRX behavior [20-21]. DRX is a softening mechanism, which strongly depends on diffusion of atoms, grain boundaries migration and dislocation density. It is reported that for higher deformation temperature and lower strain rate conditions, peak stress is observed because hardening becomes less significant [22]. In Fig. 1a, flow curves corresponding to temperature range of 1000-1100°C, exhibit peak flow stress followed by a flow softening stage and subsequently a steady state. This is because higher temperatures can offer higher mobility to the grain boundary and lower strain rate provides sufficient time for the dislocation annihilation, nucleation of new grains and growth [16]. However, the flow curves corresponding to 900-950°C do not show a notable peak flow stress and flow softening. Similar behaviour is represented in Fig. 1b for true strain of 1 s−1 in the temperature range of 900-950°C. It is reported that steady state can be absent at higher strain rate and lower strain rate conditions, which can be attributed to the occurrence of instability [21]. Zhang et al., 2011 [6], reported that DRV still exists at higher deformation temperature and at higher strain rate conditions, which affect the plastic deformation behaviour of the specimens. Constitutive equations are developed to establish the relationship between flow stress, strain rate and deformation temperature. Material constants are determined from the average slopes values of curves given in Fig. 2, and their variation with respect to true strain is presented in Fig. 3. The average value of Q is obtained as 403 kJmol−1 for 16Cr-5Ni MSS in present study. The values of Q for other stainless steels such as austenitic grades of 304 and 316 are reported as 400 kJmol−1 and 460 kJmol−1, respectively [23-24]. Values of Q for martensitic grades such as AISI 410 and 00Cr13Ni5Mo2 super martensitic stainless steel reported in earlier studies are 448 kJmol−1 and 439 kJmol−1, respectively [4, 25]. This shows that the value of Q for 16Cr-5Ni MSS obtained in the present work is comparable to that of the other grades of stainless steels. It is reported that the values of Z can establish the validity of constitutive equation [26]. Flow stress increases with the increase of Z with linear regression correlation coefficient (R) of 0.994, which indicates the degree of accuracy of equation (1) obtained in this work to represent the hot deformation behaviour of the 16Cr-5Ni MSS. Processing map is beneficial to predict the specific optimum TMP conditions. Large values of power dissipation efficiency and m obtained in processing map are associated with better workability, which can be chosen as optimum condition during the hot deformation [27]. This characteristic promotes the occurrence of DRX during the hot deformation [17-19]. TMP specimen processed at strain rate of 0.01 s−1 and in the deformation temperature range of 1000 to 1050°C exhibits the characteristics DRX because in these conditions processing map shows the highest ɳ of 31%. Hence, from the processing map two safe working regimes of (900-1100°C, 0.001-0.2 s−1), and (1025-1085°C, 0.5-10 s−1) are identified. It is reported that the possibilities of DRV and DRX are more in the safe regimes [18]. In this study, the instability occurred at higher strain rates and low temperature conditions. These also show lower values of m that are close to zero and lower efficiency of power dissipation. The optical microstructures of ASR 16Cr-5Ni MSS are shown in Figures 7a and 7b. Fig. 7a shows presence of delta ferrite and martensite. TMP specimen processed at 1000°C and 1050°C using strain rate of 0.01 s−1 revealed

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equiaxed austenite grains, which exhibits the characteristics of DRX. These microstructure regions also lie under the highest ɳ obtained in processing map.

Fig. 7. Initial microstructure of ASR 16Cr-5Ni MSS showing (a) martensite and delta ferrite phases; and (b) prior austenite grain size; and TMP specimens showing prior austenite grains in stable region using strain rate of 0.01 s−1 at (c) 1000°C; (d) 1050°C; and in unstable region using strain rate of 1 s−1 at (e) 950°C; and (f) 1000°C processing temperatures. The microstructures from the unstable region at the temperatures of 950°C and 1000°C with strain rate of 1 s−1 are shown in Figs. 7e and 7f, respectively. In these conditions the microstructures reveal very fine grains between the serrated grain boundaries. This is an inhomogeneous microstructure i.e. grain is not fully

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recrystallized and exhibits shear bands (arrow in Figs. 7e-d) resulting from localized flow of material. Flow instability leads to adiabatic shear band formation at high strain rates [28]. Flow localization is undesirable for obtaining reliable mechanical properties [29-30]. Therefore, the TMP should not be performed in the unstable regions. 5. Conclusions  The characteristics of 16Cr-5Ni MSS after the TMP have been studied in strain rate range of (0.001-10 s−1) and temperature range of (900-1100°C). DRX behaviour is observed in the specimens TMPed at strain rate of 0.01 s−1 in the temperature range of 1000-1050°C. Material constants have been determined for 16Cr-5Ni MSS using the constitutive equations and average value of Q was found to be 403 KJ/mol−1.  The Zenner-Hollomon parameter is established for 16Cr-5Ni MSS at true strain of 0.5 using the constitutive equation, which is expressed as: Z = ε exp

408000 = 1.99Χ10 8.314T

sinh 0.0082σ

.

 Safe regimes were identified at the TMP conditions of (900-1100°C, 0.001-0.2 s−1), and (1025-1085°C, 0.5-10 s−1). Equiaxed austenite grains were obtained for the TMP condition at strain rates of 0.01 s−1 and temperatures of 1000°C and 1050°C. Two instability regions are identified for 16Cr-5Ni MSS- First region lies in the strain rate range of 0.3-10 s−1 and temperature range of 900-1020°C. The second lies in the strain rate range of 0.5-10 s−1 at temperature of 1100°C. In the unstable regimes, localized shear bands were seen in the microstructure obtained. Acknowledgements The authors are grateful to the Department of Science and Technology, Ministry of Science and Technology, Government of India (Grant no. SR/S3/ME/0029/2009-(G)) for the research funding. References [1] Niederau HJ; A new low-carbon 16Cr-5Ni stainless martensitic cast steel: Stainless steel casting; A symposium sponsored by ASTM committee, 1980, p 382-393. [2] Wen DC; Improvement of slurry erosion resistance of martensitic/ferritic duplex stainless steel by hot rolling; Metals and Materials International Journal , 2010, Vol. 16 p 13-19. [3] Xiao YH, Guo C; Constitutive modelling for high temperature behavior of Cr12Ni3Mo2VNbN martensitic steel; Materials Science and Engineering A, 2011, Vol. 528 p 5081-5087. [4] Momeni A, Dehghani K.; Characterization of hot deformation behavior of 410 martensitic stainless steel using constitutive equations and processing maps; Materials Science and Engineering A, 2010, Vol. 527 p 5467-5473. [5] Ren F, Chen F, Chen J; Investigation on dynamic recrystallization behavior of martensitic stainless steel; Advances in Materials Science and Engineering, 2014, Vol. 2014 p 1-16. [6] Zhang L, Li Z, Lei Q, Qiu WT, Luo HT; Hot deformation behavior of Cu–8.0Ni–1.8Si–0.15Mg alloy; Materials Science and Engineering A, 2011, Vol. 528 p 1641-1647. [7] Raj R; Development of a processing map for use in warm-forming and hot-forming processes; Metallurgical and Materials Transactions A, 1981, Vol. 12 p 1089-1097. [8] Prasad YVRK; Recent advances in the science of mechanical processing; Indian Journal of Technology, 1990, Vol. 28 p 435-451. [9] Prasad YVRK, Seshacharyulu T; Processing maps for hot working of titanium alloys; Materials Science and Engineering A, 1998, Vol. 243 p 82-88.

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